1. Field of The Invention
The present invention relates generally to an acceleration sensor for automotive vehicles. Particularly, the invention relates to a semiconductor type acceleration sensor, a detector chip of which may be protected from excess input in an acceleration direction.
2. Description of The Prior Art
Semiconductor type acceleration sensors are well known in the art and have various applications in the automotive field including suspension control, air bag system control and the like.
Referring to FIG. 8, a detector 55 chip of such semiconductor type acceleration sensors is generally manufactured by LSI techniques on a silicon substrate 51. As seen in the drawing, the detector chip 55 is formed with a heavy bob section (i.e. a pendulum section) 52 of cantilever shape by etching processing, or the like, Also, a strain gauge section 53 comprising a piezo resistance element, for example, is formed on a root portion of the heavy bob section 52, and at the same time, a signal processing circuit 54, for example, an amplifier circuit and so forth is assembled on the silicon substrate 51 on which the heavy bob portion has been formed. Thereby, a detector chip 55 capable of being a principle component of an acceleration sensor may be formed with a convenient, single-chip construction.
According to the above-described construction, an electric signal output is obtained based on a variation of strain produced in the strain gauge portion 53 when acceleration is applied to the heavy bob section 52.
Referring now to FIG. 9, such a detector chip 55 as described above, is sealed in a casing 57 filled with insulating silicone oil 58 or the like to comprise a detection unit 58 of an acceleration sensor. As seen in FIGS. 10 and 11, the detection unit 58 is mounted on a substrate 80 with processing circuits 59 and the substrate 60 is incorporated into a housing 61. A sealing agent 62 fills a rear side of the housing 61 and a connector 63 is further provided.
In the above-described construction, silicon oil 56 is provided inside the casing 57 so as to provide a damping force, in a case where an excess acceleration is applied to the detect chip 55, for preventing weakening and/or deformation of a root portion of the heavy bob section 52.
The silicon oil 56 provides such damping characteristics according to its viscous resistance; however, the viscosity of the silicon oil 56 is largely varied according to temperature. Therefore detection sensitivity of the detector chip 55 is correspondingly varied and the consistency and accuracy of the acceleration sensor is degraded.
In addition, aside from the problem of viscosity variation of the silicon oil 56, the presence of a damping fluid in the casing 57 of the detection unit 58 reduces the detection sensitivity of the detector chip 55 because an absolute displacement of the heavy bob 52 is suppressed by the viscous resistance of the silicon oil 56, or other damping fluid. This requires that the sensitivity and stability, particularly of an amplifier circuit and filter circuit as well as other signal processing circuits associated with the acceleration sensor, be substantially higher. Thus the cost and complexity of the entire sensor is increased.